Wavelength division multiplexed (WDM) passive optical networks (PON) dramatically increase the link capacity compared with other optical fibre access networks. Using dedicated wavelengths for each connection between a subscriber and the central office, the current technology allows up to 40 10 Gigabit Ethernet (GbE) links to operate on the same optical fibre, exploiting only C and L bands, with an increase in bandwidth expected in the near future. The bandwidth combined with the end to end connectivity offered by each wavelength (virtual point to point) can be used for ultra high speed access, and for high value and low cost mobile backhauling.
Several system architectures and transmission techniques have been proposed to lower the cost or increase the performance of a WDM PON. A common feature of these architectures and techniques is the use of a “colourless” optical network termination (ONT), that is an ONT which can be configured to operate at any network wavelength at the subscriber side, and can therefore be connected to any port of the remote node. The use of colourless ONTs allows higher production volumes, a low number of spare parts, and makes it easier for network operators to manage an unpredictable network evolution, where different users subscribe or unsubscribe in a random sequence.
There are basically three ways to implement a colourless ONT: sending the optical carrier to be used for uplink transmission to the ONT from a remote location, typically the Central Office (CO); reusing part of the downlink optical signal to as the uplink optical carrier; and locally generating the uplink optical carrier using a tunable laser located in the ONT.
WDM PONs not based on tunable lasers at the ONTs are cheaper, as general rule, but offer poor performance due to link attenuation (the optical carrier goes from the CO to the ONT before being transmitted back again) and cross-talk between upstream and downstream signals, caused by reflections between uplink and downlink.
Using tunable lasers to generate the upstream carrier signal at the ONT solves both of these issues because different wavelengths are allocated for the downstream channels and the upstream channels. Although tunable lasers are quite expensive today, the technology is evolving so that they appear to be the most promising solution in the medium to long term. The need to keep an inventory of ONTs supporting all possible wavelengths could be avoided through dedicated protocols or operations and maintenance (O&M) procedures, but in this case one of the main benefits of WDM-PON connectivity, its protocol transparency, is lost.
A method of configuring a transmission wavelength of an optical transmitter in an optical network using physical layer signaling is described in WO2011/020503.